cli: allow option groups to force async/sync operations

Some operations are clearly asynchronous (e.g. monitoring modem state), while
others are clearly synchronous (e.g. printing modem info). So just allow option
groups to force the operation to be async or sync based on the detected action.

cli/mmcli-manager.c

@@ -98,6 +98,9 @@ mmcli_manager_options_enabled (void)
         exit (EXIT_FAILURE);
     }
 
+    if (monitor_modems_flag)
+        mmcli_force_async_operation ();
+
     return !!n_actions;
 }